Epitaxy, Ultrathin Films

Usually, one speaks of epitaxial growth in cases where the stmcture of a film grown from the gas phase is essentially determined by the structure of the substrate. Epitaxy in the precise sense means that the crystal structure of the substrate is continued within the growing fUm. This is possible when the crystal stractures of the substrate and the growing film are commensurate. Examples are shown in Eigs. 1.9 and 2.23. Frequently, one refers also to Organic Molecular-Beam Epitaxy, OMBE. 

The preparation of well-defined thin or ultrathin crystalline films by sublimation or molecular-beam epitaxy is a technology which depends upon a large number of experimental parameters, which we cannot treat in detail here. Ultrahigh vacuum is in any case a prerequisite. More on this subject can be found for example in the review articles by Forrest [18]. 

The preparation of ultrathin films by the Langmuir-Blodgett method was already treated in Sect. 2.4. It is, to be sure, obviously very difficult to obtain films without 

An important goal is the preparation of films which are as far as possible free of defects, e.g. pinholes, which could lead to short-circuits in electrical measurements. The films should be sufficiently stable that electrodes (e.g. gold) can be attached to their free surfaces without the electrode material being incorporated into the film. 

for example, alkane thioles or alkane silanes are used, then the thickness of the film is determined by the length of the molecules. With up to 20 CH2 groups, film thicknesses of some nm are obtained. At the ends of the molecules pointing away from the substrate, various end groups can be attached. For example, with aromatic end groups and their mutual tt-tt interactions, the molecular film can be stabilised and sealed off More details can be found e.g. in the review article by Schreiber [20], and in [19]. 

---

A. Schmidt, L. K. Chau, A. Back, and N. R. Armstrong, Epitaxial Phthalocyanine Ultrathin Films Grown by Organic Molecular Beam Epitaxy (OMBE), in Phthalo-cyanines, Vol. 4, C. Leznof and A. P. B. Lever, eds., VCH Publications, 1996. 

With appropriately substituted oxetanes, aluminum-based initiators (321) impose a degree of microstmctural control on the substituted polyoxetane stmcture that is not obtainable with a pure cationic system. A polymer having largely the stmcture of poly(3-hydroxyoxetane) has been obtained from an anionic rearrangement polymerisation of glycidol or its trimethylsilyl ether, both oxirane monomers (322). Polymerisation-induced epitaxy can produce ultrathin films of highly oriented POX molecules on, for instance, graphite (323). Theoretical studies on the cationic polymerisation mechanism of oxetanes have been made (324—326). 

Lai B-K, Kornev lA, Bellaiche L, Salamo GJ (2005) Phase diagrams of epitaxial BaTi03 ultrathin films from first principles. Appl Phys Lett 86 132904... 

Perhaps the greatest disadvantage of CSD is the difficulty in depositing ultrathin films (thickness <30 nm). While the possibility has been demonstrated in selected systems, the method is not usually viewed as amenable to that thickness requirement. Another disadvantage of CSD is the fact that three-dimensioanl structures with high aspect ratios cannot be conformally coated. Lastly, while epitaxial films have been prepared by the method, ° generally the films produced are polycrystalline and often not oriented. 

This book shows that the initial stages of 2D and 3D metal phase formation under electrochemical conditions are, in general, well developed and understood on an atomic level. The knowledge of the substrate surface properties is necessary for a well-defined preparation of 2D and 3D metal phases, surface alloys, ultrathin films, and heterostructures. The structural and epitaxial behavior of metal deposits determines their physical and chemical properties. 

Ultrathin Films. The demand for artificial materials or so-called designer solids is increasing. Many applications such as solid state lasers and new generations of transistors require ever finer structuring of materials. It is very common for the properties of devices based on heterostructures to depend on the quality of the interfaces. The structures can be grown by chemical vapor deposition (CVD) or molecular beam epitaxy (MBE). Transmission electron microscopy... 

The investigation on the electrochemical epitaxial growth of palladium on gold and platinum singlemonolayer coverage of palladium on Pt(lll) electrode by the immersion technique for the first time [74]. Llorca and coworkers investigated the irreversibly adsorbed palladium on Pt(hiJ) in acidic solution [75] and reported that electrocatalytic activity for the oxidation of formic acid on the Pt(lOO) electrodes vras improved drastically by the palladium-adlayer modification, while that on the Pt(lll) electrode was not greatly affected by the Pd-modification [76]. However, it is difficult to prepare an ultrathin film of palladium with various thickness by the immersion technique, and STM observation of the atomic structure of these surfaces is not available yet. 

Jia, C.-L., Nagarajan, V, He, J.-Q., Houben, L., Zhao, T, Ramesh, R., Urban, K., Waser, R. Unit-cell scale mapping of ferroelectricity and tetragonality in epitaxial ultrathin ferroelectric films. Nat. Mater. 6,64-69 (2007)... 

The aim of this chapter is to report on recent advances in the in situ Mossbauer spectroscopy with synchrotron radiation on thin films that became possible due to the instrumentation developments at the nuclear resonance beamline ID 18 of the ESRF. After a detailed description of the beamline and of the UHV system for in situ experiments, a brief introduction into the basic NRS techniques is given. Finally, the application of these techniques to investigate magnetic, diffusion, and lattice dynamics phenomena in ultrathin epitaxial Fe films deposited on a W(l 10) substrate is presented and discussed. 

Schmidt A, Chau LK, Back A, Armstrong NR (1996) Epitaxial phthalocyanine ultrathin films grown by organic molecular beam epitaxy (OMBE). In Leznoff CC, Lever ABP (eds) Phthalocyanines properties and applications, vol 4. VCH, New York, Weinheim, Cambridge, pp 307-341... 

ALD, also known as atomic layer epitaxy (ALE), has gained more and more attention for ultrathin film deposition for applications in many fields such as optics and... 

C.M. Schneider, P. Bressler, P. Schuster, J. Kirschner, J.J. Demiguel, R. Miranda, Curie-temperature of ultrathin films of fee cobalt epitaxially grown on atomically flat Cu(lOO) surfaces. Phys. Rev. Lett. 64(9), 1059-1062 (1990)... 

---